The early discovery of hidden defects in parts and products is of increasing concern to manufacturers as they strive to obtain superior product quality. Particularly, there is a need for the early discovery of defects which could remain latent or undiscovered for an indeterminate time. There is also a need in the field of electronic circuits to determine properties of components and parts, such as capacitors, which may be predictive of a lack of long term reliability, even though not ordinarily indicative of defects.
Thermography, or thermal analysis, has attracted considerable attention as one way of discovering such defects or properties of parts. All objects "glow" from thermal radiation with an intensity and "color" which is dependent upon temperature. At room temperature this color is within a range known as infrared and cannot be seen with the unaided eye. At extreme temperatures an object will glow visibly as in the case of iron heated in a fire. This thermally radiative property of objects can be used to measure the temperature of an object's surface without need for any kind of contact. Any of several types of equipment can convert this temperature information into a black and white or color image that represents the temperatures within the scene. Such equipment can be called a "thermal imager" and can be used to study non-visible properties of electronic assemblies in the hope of locating defective devices or devices which may eventually prove to be less reliable than others.
Capacitors, in particular, are exceedingly difficult to test, or "screen", in the production quantities that are commercially necessary because they tend to have small variations in lead resistances and other properties which are hard to test because the capacitor conducts electricity only transiently as it charges or discharges. One consequence of that fact is that capacitors are not easily heated for thermographic testing.
Moreover, it is a property of modern electronic logic circuits that large numbers of small capacitors, e.g., in the range from 0.0 1 .mu.f to 1.0 .mu.f, are used in circuits primarily for the purpose of protecting the active devices, such as integrated circuits and the transistors and other components therein, from transients. These capacitors are known as transient decoupling capacitors, as well as by other names. The result in a typical integrated logic circuit is that there are a large number of such transient decoupling capacitors. All such capacitors reach fairly directly to the power supply in their connections; but it is a property of any such decoupling capacitor that, if it is to be useful, it must be immediately adjacent to the transistor which it is intended to protect so that the varying lengths of leads will not support transients that will damage the transistor. Nevertheless, despite the varying lead lengths from the power supply to such transient decoupling capacitors, these capacitors are essentially connected in parallel with one another. This makes the testing of the completed circuit board even more difficult because conventional test methods are unable to discern individually defective capacitors when connected in parallel (with the exception of direct shorts).
In the section entitled "Thermography", in Evaluation Engineering, December 1988, particularly in the article entitled "Understanding the Expanded Role of Thermal Imagers in Production Testing" by Hugh Danaher, starting at page 74, it is explained that thermal imaging can be very useful in screening integrated circuits, each of which has multiple such capacitors. At page 77, it is stated:
An alternate test method is to apply a high-frequency (10 kHz), low-voltage (0.5V) ac supply which will cause all capacitors to heat. Using this approach, all capacitor conditions can be determined. Open capacitors will not heat, shorted capacitors will heat excessively and out-of-spec capacitors will exhibit different thermal characteristics than the accepted standard.
Our experimental efforts to duplicate the above-described testing technique indicates the technique as described is primarily useful for testing individual capacitors or small numbers of capacitors of relatively exceptional characteristics, such as electrolytic capacitors. The technique of the article is presumably applied by supplying the AC stimulus to the capacitors by clip leads or hand held probes.
It appears that the technique of the article cannot achieve successful stimulation of multiple capacitors of more typical characteristics (small capacitance) to increase their temperatures by even as little as 0.1.degree. C. Yet testing of transient decoupling capacitors and other capacitors of relatively common or typical characteristics after they are connected into an electronic logic circuit would be of great value because new or latent weaknesses of the capacitors may be developed or revealed as a result of the processing that includes them in the integrated circuit.